Validate mkdocs

[fyliu]

Fixes #536

What changes did you make?

• configured mkdocs validation for maximum strictness
• added script to validate mkdocs
• checked in fix for validation warning
• added GHA workflow to validate mkdocs
• added documentation for the GHA

Why did you make the changes (we will use this info to test)?

• we need the validation to check the mkdocs build to make sure the mkdocs site deployment works and is free of basic issues like broken internal links

Testing

1. validate-mkdocs.sh - Just run it to test. Maybe re-introduce the validation issue that's been fixed.

2. GHA workflow - use act to test.

   1. Install act.

   2. Run workflow by file.

      act -W '.github/workflows/pull-request.yml'

   3. Can introduce the validation issue to see it fail.

[fyliu]

I borrowed the workflow from the docker-mkdocs project and it failed because we don't install mkdocs in our project. I'll see if I can make use of the docker image from there.

[fyliu]

I modified the GHA to use the image from hackforlaops/mkdocs:latest. /dev/null -> /tmp because GHA doesn't like /dev/null

[del9ra]

Thanks for contributing, Fang! I tested your code on my PC and it works perfectly.